1. Field of the Invention
The present invention relates generally to a figure-8 optical fiber pulse laser using a dispersion imbalanced nonlinear optical loop mirror (DI-NOLM). More particularly, it relates to a new type of the figure-8 optical fiber pulse laser generating stable femto-second optical pulse trains.
2. Description of the Related Art
Generally, the mode-lock method of this laser is a skill to produce a pulse train of narrow pulse width using a laser, and it is divided into the active mode-lock method and the passive one. The conventional figure-8 optical fiber laser is one of the passively mode-locked optical fiber lasers. It may produce a femto-second optical pulse train, but it has a difficulty to produce stable pulse trains. In order to improve the switching effect of a nonlinear optical loop mirror (NOLM) as a switch of the figure-8 optical fiber laser, an optical amplifier is added into the NOLM to form a nonlinear amplifying loop mirror (NALM). The NALM considerably improves the switching effect of the NOLM.
FIG. 1 shows a schematic diagram for the conventional figure-8 optical fiber laser. The nonlinear loop part comprises a pump LD (laser diode) (10), a WDM (wavelength division multiplexing mirror) coupler (15), an EDF (erbium doped optical fiber) (20), a DSF (dispersion shifted fiber) (25), and a PC (polarization controller) (30). On the other hand, the linear loop part comprises a PC (30), a BPF (optical band pass filter) (40), an optical isolator (45), and an output coupler (50). As shown in FIG. 1, the figure-8 optical fiber laser is composed of the linear loop part and the nonlinear part switching the optical pulse train. Here, the optical gain is achieved through the EDF (20), and the switching part is composed of the NALM.
The optical amplifier, EDF (20), is not located at the center of the NALM but lopsided. The optical pulses inputted to the NALM are divided into two through the optical coupler (35). One of them propagates the loop clockwise, and it first passes the optical amplifier, EDF (20), and then, DSF (25). While, the other propagates the loop counterclockwise, and it passes DSF (25), and then EDF (20). Since the nonlinear phase shift of the clockwise pulse is greater than that of the counterclockwise pulse, the self-switching phenomena occur when two pulses meet at the optical coupler (35). The width of the self-switched pulse becomes narrower, and the pulse is amplified again at the EDF (20). Accordingly, the figure-8 optical fiber laser produces the narrow optical pulses.
However, it is difficult to produce the stable pulse train due to the noise and the stimulated Brillouin scattering. The noise is originated from the bidirectional gain difference of the optical amplifier, and the stimulated Brillouin scattering with nonlinear characteristic is from the bidirectional pulses propagating the optical amplifier.
The present invention is contrived in order to solve the above problem. It is an object of the present invention to provide a figure-8 optical fiber pulse laser with a dispersion imbalanced nonlinear optical fiber loop mirror (DI-NOLM). The laser according to this invention produces stable femto-second optical pulse trains using DI-NOLM.
In order to attain the above object, the optical amplifier is not included in the NOLM, instead, it is located at the unidirectional linear loop part. And the noise due to the bidirectional gain difference of the optical amplifier is reduced. Two optical fibers of different dispersion values are used at DI-NOLM. Therefore, present invention provides a figure-8 optical fiber pulse laser with a dispersion imbalanced nonlinear optical fiber loop mirror (DI-NOLM) improving the switching characteristic.